An electrophotographic image forming apparatus has good reproducibility and operability, and can form a high-definition image at low cost. Therefore, such an image forming apparatus is used in many ways, for example, as a copying machine, a printer, a facsimile, and a multifunction printer that has two or more functions of the copying machine, the printer and the facsimile. The electrophotographic image forming apparatus includes a photoreceptor, a charging device, an exposure device, a developing device, a transfer device and a fixing device. The photoreceptor is a member on which surface an electrostatic latent image is formed in accordance with image information. The charging device is a member that charges the surface of the photoreceptor. The exposure device is a member which emits signal light to the charged photoreceptor surface, to form the electrostatic latent image on the photoreceptor surface. The developing device is a member which supplies toner to the electrostatic latent image formed on the photoreceptor surface, to form a toner image on the photoreceptor surface. The transfer device is a member that transfers the toner image formed on the photoreceptor surface to a sheet (recording medium). The fixing device is a member that fixes, to the sheet, the toner image transferred to the sheet.
A roller device including a fixing roller and a pressure roller is generally used as the fixing device. The fixing roller (heating roller) includes a heater (heating section). The pressure roller is provided by being in contact with and applying pressure to the fixing roller; due to elastic deformation of an elastic layer in the pressure roller, a nip section is formed between the fixing roller and the pressure roller. The fixing device that employs the fixing roller (heating roller) sandwiches, at the nip section between the fixing roller and the pressure roller, a sheet on which an unfixed toner image is formed. This sheet is carried by rotating the fixing roller and the pressure roller; meanwhile, the toner image formed on the sheet is melted by the heat of the surface of the fixing roller and is fixed to the sheet. Further, the pressure roller causes the sheet to be pressed against the fixing roller, which pressure reinforces the fixing of the toner image to the sheet.
Recently, demands have been increasing for attaining energy efficiency with the fixing device in terms of environmental protection, and therefore a fixing device with a reduced warm-up period has been actively developed. The following methods are some examples of how to reduce the warm-up period: attaining high-efficiency in generation of electric heat (heat exchange) with a heater inside the fixing roller; reducing heat capacity of the fixing roller; and the like. A fixing roller used for monochrome printing is generally made up of a metal core bar whose periphery is covered with fluorinated resin or a fluorinated tube to obtain toner releasability, and has a heat source (heater) such as a halogen lamp provided inside the core bar. Further, as disclosed in, for example, Patent Literatures 2 and 3, the pressure roller has a relatively thick elastic layer made of a heat-resistant rubber or sponge formed around the metal core bar, which elastic layer is covered with fluorinated resin or a fluorinated tube to attain the toner releasability. The pressure roller having such a configuration is pressed against the fixing roller, thereby causing elastic deformation of the pressure roller. As a result, a fixing nip is formed, where toner is fixed onto the sheet.
Further, a fixing roller having the following configuration is used: an elastic layer made of, for example, silicon rubber, is formed around a core bar, and a release layer is formed as a surface layer on a peripheral surface of the elastic layer. A fixing device that employs the fixing roller having such a configuration is disclosed in, for example, Patent Literature 1 or 4 as a fixing device used for color printing. The fixing device used for color printing employs a fixing roller that has a relatively thick elastic layer whose thickness is of a few millimeters. This causes delay in an initiation period or the like, contrary to attaining energy efficiency. However, an effect caused by the delay in the initiation period or the like is reduced by reducing thickness of the elastic layer. Further, the elastic layer made of silicon rubber serves as a heat insulating layer. This can prevent excessive heat transfer from the fixing roller via a fixing nip section to the pressure roller, and therefore improves temperature following capability. This also improves adhesiveness between a surface of the fixing roller and a sheet surface, thereby improving fixing capability.